Castamir "The Usurper" (Third Age 1259 – 1447, aged 188 years) usurped the throne of Gondor from Eldacar, ruling from Third Age 1437 to 1447, until he was violently deposed.

The great-grandson of the unremarkable king Calmacil, Castamir was not in line for the throne, but his royal background had made him the "Lord of Ships" under the previous king, Valacar. His lineage and influence left him in a position to challenge the current king, Eldacar, who was only half Gondorian. As King Valacar's only son, Eldacar was the legitimate heir to the throne, but his half Rhovanion lineage angered citizens who believed that pure Númenórean blood must be maintained. Capitalizing on what became outright protest against the current king, Castamir was able to incite rebellion and besiege Eldacar within the city Osgiliath in 1437 of the Third Age. The city was set on fire by Castamir's troops, destroying the famed Dome of Stars. Shortly thereafter, Eldacar fled the city and into exile in Rhovanion, to the north. With the defeat of the king, Castamir seized the throne.

For ten years Castamir ruled Gondor with an iron fist, while Eldacar quietly regained power. Castamir ruled cruelly, and committed many foul deeds, including executing Eldacar's own son, Ornendil. At first the people of Gondor tolerated Castamir because of his pure Númenórean blood. Then, in 1447, after a decade under the wicked rule of the Usurper (as Castamir was now called), the people of Gondor rallied around the true king, Eldacar. Castamir was slain by Eldacar's forces at the Battle of Erui, the decisive battle of the Kin-strife, as this period came to be known. At the Battle of Erui, Eldacar personally slew Castamir.

Castamir's influence did not end there. Remnants of his forces, and his surviving sons, retreated from the Battle of Erui to the port of Pelargir and escaped by ship to the Haven of Umbar, far to the south. These descendants of Castamir became the so called "Corsairs of Umbar", fierce pirates who terrorized the ports of Gondor with the help of the men of Harad. They were perhaps the greatest threat to Gondor for nearly 400 years, commanding massive fleets of ships. There most infamous exploit was the Raid of Pelargir in 1634, in which a Corsair armada sailed up the river Anduin to the port of Pelagar, deep in Gondorian territory. This raid coincided with the young king Minardil's visit, and he was slain in by the Corsairs as they attacked.

They remained a significant threat until 1810, when King Telumehtar retook Umbar for Gondor. In this battle the last descendants of Castamir were finally killed. So great a victory was this, that forever after Telumehtar was known as Umbardacil, which literally means "victor over Umbar".